Hardmetals are to be regarded as a composite material wherein the monotungsten carbide (WC) phase acts as the hardness carrier. The brittleness of pure WC which is associated with the high degree of hardness is compensated for by the metallic binder phase (generally Co but also Fe and Ni and alloys thereof and optionally Cr). In the event of high binder contents, the mechanical abrasion resistance is not sufficient, and with low binder contents the mechanical strength obtained is inadequate. In practice, therefore, hardmetals are composed of 3 to 30 wt. % of Co, values of 5 to 12 wt. % being encountered most often. For a given binder content, hardness and toughness can be adjusted by way of the degree of dispersion of the WC phase in the sintered hardmetal (known as the Hall-Petch relation). The range of variation of the tungsten carbide powders used for this purpose is from 0.4 xcexcm to 50 xcexcm FSSS (ASTM B 330). The hardnesses which can be obtained in the sintered hardmetal are from 1950 to 850 kg/mm2 HV30 (Vickers hardness with a 30 kg load) with 10 wt. % of Co.
The object of the invention is to extend downwards the available hardness range of hardmetals with the same binder content. The background is that the specific wear behavior of hardmetal tools during cutting-type rock machining is evidently so that a monocrystalline structure which is as coarse as possible in the sintered hardmetal permits particularly long edge lives and hence high cutting rates without premature tool failure. Such applications are found in tunnel driving, borehole drilling and in mining (EP 0 819 777 A1).
The industrially established method of preparing tungsten carbide is the reduction of WO3, tungstic acid or ammonium para-tungstate under hydrogen, initially to the metal powder followed by carburization. The tungsten metal powder obtained in this way is obtained in particle sizes from 0.3 xcexcm to 50 xcexcm, and the particle size may be adjusted within certain limits by means of the charge amount or bed height, moisture content of the hydrogen, reduction temperature and residence time. During the subsequent carburization of the tungsten metal powder thus obtained with solid carbon at temperatures of up to 2,200xc2x0 C., this particle size remains substantially unchanged after work up. In order to obtain average particle sizes above about 12 xcexcm FSSS, the range that can be achieved in the above production parameters during reduction is no longer sufficient.
U.S. Pat. No. 4,402,737 describes doping tungsten oxide with alkali chlorides, particularly with LiCl with which the highest FSSS values in the tungsten metal powder can be obtained. Polycrystalline powders are the development objective, however (column 1, line 22). Such tungsten metal powders, after carburization, lead to agglomerated composites of fine WC crystals which in turn leads to a higher degree of dispersion of the WC phase in the sintered hardmetal and hence to a comparatively high degree of hardness. Another disadvantage is that the gaseous HCl formed during reduction with hydrogen leads to increased corrosion in the plants used to separate the hydrogen from the water formed.
According to EP 0 819 777 A1, the fractionated classification of WC powders with a broad particle size distribution is proposed for the preparation of coarse, monocrystalline WC powders, e.g., by grinding followed by air classification. This separation can be achieved with sufficient accuracy and also leads to correspondingly satisfactory hardmetals with low hardnesses, but it entails a large amount of equipment and the production of coupled products. It is claimed that with this process (and an unconventional mixing of WC and cobalt metal powder by means of a complex pyrolysis process instead of conventional crushing and grinding) it is possible to obtain hardmetals which, with 6% of Co, have a Vickers hardness of 980 kg/mm2 (EP 0 819 777 A1) and correspondingly good properties.
The object of the invention is to provide a method of preparation with a high yield for monocrystalline WC with an average particle size greater than 50 xcexcm FSSS and sufficient purity, wherein the hardmetals produced in a correspondingly conventional manner (i.e. by crushing and grinding cobalt metal powder with the tungsten carbide) with a narrow particle size distribution of the WC should have a hardness of at most 850 HV30 with 10 wt. % of Co or 980 HV30 with 6% of Co. To this end, it is necessary to prepare a coarse-crystalline tungsten metal powder in the first instance.
The invention relates to a process comprising reducing a component selected from the group consisting of tungsten oxide powders and molybdenum oxide powders, in the presence of alkali metal compounds, and preparing tungsten powder, molybdenum powder, mixtures thereof, or a carbide; wherein at least two alkali metal compounds are used in a ratio so that mixed alkali tungstate or molybdate formed in an intermediate step ((Li, Na, K)2 WOZ, (Li, Na, K)2MoOZ) has a melting point of less than about 550xc2x0 C., wherein the value of z is from 3 to 4. The invention also relates to a tungsten metal powder, a molybdenum metal powder, a tungsten carbide powder made by such a process. In one embodiment, the invention relates to a tungsten carbide powder with an average particle size of  greater than 50 xcexcm FSSS.